Re: [PATCH v6 1/6] arm64: ptrace: add PTRACE_SET_SYSCALL

Will Deacon
2014-08-26T17:46:59Z

On Fri, Aug 22, 2014 at 01:19:13AM +0100, AKASHI Takahiro wrote:
> On 08/22/2014 01:47 AM, Kees Cook wrote:
> > On Thu, Aug 21, 2014 at 3:56 AM, AKASHI Takahiro
> > <takahiro.akashi@linaro.org> wrote:
> >> To allow tracer to be able to change/skip a system call by re-writing
> >> a syscall number, there are several approaches:
> >>
> >> (1) modify x8 register with ptrace(PTRACE_SETREGSET), and handle this case
> >> later on in syscall_trace_enter(), or
> >> (2) support ptrace(PTRACE_SET_SYSCALL) as on arm
> >>
> >> Thinking of the fact that user_pt_regs doesn't expose 'syscallno' to
> >> tracer as well as that secure_computing() expects a changed syscall number
> >> to be visible, especially case of -1, before this function returns in
> >> syscall_trace_enter(), we'd better take (2).
> >>
> >> Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
> >
> > Thanks, I like having this on both arm and arm64.
>
> Yeah, having this simplified the code of syscall_trace_enter() a bit, but
> also imposes some restriction on arm64, too.
>
> > I wonder if other archs should add this option too.
>
> Do you think so? I assumed that SET_SYSCALL is to be avoided if possible.
>
> I also think that SET_SYSCALL should take an extra argument for a return value
> just in case of -1 (or we have SKIP_SYSCALL?).

I think we should propose this as a new request in the generic ptrace code.
We can have an architecture-hook for actually setting the syscall, and allow
architectures to define their own implementation of the request so they can
be moved over one by one.

Will